docs: clarify scope of config file and environment variables

I got pretty confused about why I can't set auth.method from the config
file (and also why there were no docs saying what the JSON/YAML/etc key
would even be).

Document that this doesn't work to try to prevent future confusion here.

Caveat: I wrote this documentation by hand, but I did not actually read
the code carefully enough to have figured this out myself. I am
trusting the claims made by an LLM coding agent (which do at least seem
to match empirical evidence).

cmd/root.go

@@ -135,6 +135,11 @@ file named .filebrowser.{json, toml, yaml, yml} in the following directories:
 - $HOME/
 - /etc/filebrowser/
 
+**Note:** Only the options listed below can be set via the config file or
+environment variables. Other configuration options live exclusively in the
+database and so they must be set by the "config set" or "config
+import" commands.
+
 The precedence of the configuration values are as follows:
 
 - Flags

www/docs/cli/filebrowser.md

@@ -26,6 +26,11 @@ file named .filebrowser.{json, toml, yaml, yml} in the following directories:
 - $HOME/
 - /etc/filebrowser/
 
+**Note:** Only the options listed below can be set via the config file or
+environment variables. Other configuration options live exclusively in the
+database and so they must be set by the "config set" or "config
+import" commands.
+
 The precedence of the configuration values are as follows:
 
 - Flags